1. Field of the Invention
The present invention relates to a sheet conveying apparatus. More particularly, the present invention relates to a sheet conveying mechanism which is capable of preventing occurrence of deviation of registration (hereinafter called "registration deviation") in a printer.
The present application is based on Japanese Patent Applications Nos. Hei. 9-360925 and 10-356282, the content of which is incorporated hereinto by reference.
2. Description of the Related Art
Conveyance of paper in a thermal printer has been mainly performed by any one of three methods which are a grip roller drive method, a platen roller drive method and a drum drive method. Among the foregoing methods, printers adaptable to a somewhat large sheet size of about A3-size employ the grip roller method structured as shown in FIG. 15. Referring to FIG. 15, reference numeral 101 represents a platen roller disposed opposite to a thermal head 100 and arranged to rotate counterclockwise so as to convey sheets 103 and apply a predetermined pressure to the thermal head 100 so as to press the recording sheet 103 and a toner ribbon 102 guided by a guide roller 104. Moreover, a roller pair comprised of a capstan roller 105 and a pinch roller 106 forms a conveying mechanism having a large holding force so as to convey the sheet 103 from a recording portion to a discharge portion. A backup roller 107 is an adjustment roller.
When a color recording operation of a plurality of images is performed by superimposing the images in a plane sequential manner such that confirmation of registration (position alignment) is performed by using registration marks "+" as shown in FIG. 16, so-called registration deviation occurs if the sheet conveying mechanism cannot realize a required conveyance accuracy. In this case, there arises problems in that color shift and bleeding occur.
In order to ensure a required conveyance accuracy of a recording sheet conveyed in the grip roller drive method, the surface of the capstan roller 105 is devised (e.g. a roll having a random spray deposit surface) to increase the friction with the reverse side of the sheet 103. As an alternative to this, the material of the surface of the pinch roller 106 is devised to increase the friction with the right side of the sheet 103. Another contrivance has been employed in which the pressure is increased to enlarge the frictional force while the friction factor is maintained.
FIGS. 18A and 18B show the surface condition of a capstan roller having a spray deposit surface: FIG. 18A is an enlarged view of the spray deposit surface; and FIG. 18B is an enlarged sectional view thereof. As shown in FIGS. 18A and 18B, the spray deposit surface is uneven in a random fashion, which has made it unsatisfactory to ensure that a sheet of paper is surely conveyed.
An improvement of the grip belt method shown in FIG. 17 has been employed such that the pair of the rollers is changed to a belt structure. A rubber capstan belt 110 is attached around a pair of rollers 108 and 109. A rubber pinch belt 113 is attached around a pair of rollers 111 and 112. Thus, the area of contact for conveying the sheet is enlarged so as to enlarge the frictional force.
However, there is no definite method capable of improving the registration accuracy in the above described methods. The reason for this lies in that the contrivances of the grip roller method such that the surfaces of the capstan roller 105 and the pinch roller 106 are modified cannot attain a satisfactory effect.
When the pressure is increased, the shaft of the grip roller is warped, and therefore uniform pressure cannot be applied. Although the backup roller 107 may be employed or a separable comb-toothed roller may be employed to press the central portion, delicate adjustment must be performed in the above-mentioned case. Therefore, there arises a problem in that a satisfactory effect cannot be obtained.
When the frictional force is enlarged by changing the grip roller to a belt structure, the area of contact of the belt can be enlarged. However, such a structure can merely obtain almost the same effect as the structure in which the two pairs of grip rollers are provided.